A.C. electric rotary machines such as motors and generators, and linear machines such as actuators of the induction and permanent magnet synchronous type, are generally used because they are extremely rugged, reliable, easy to control, and in particular have a high torque capacity and high power density ratings. Induction machines operate on the principle that current traveling in stationary coils or windings of a stator produce a rotating magnetic field which in turn produces a current in a rotor occupying the space where the rotating magnetic field exists. The induced current in the rotor reacts with the rotating magnetic field to produce a force. To obtain a linear motion, the stator and rotor are replaced with a stator and shuttle but the operating principle is the same.
Heretofore, it was believed that there was a fundamental limit to torque density in such machines caused by the fact that the interaction in an electric machine is between two solenoidal quantities: current and flux. Flux density is limited by material considerations; current density is limited by heating, by machine reactance considerations, and by the fact that too much current density can produce tooth tip saturation from leakage flux. Therefore, it was believed since power density was limited, the only way to increase power was to increase the volume of the machine.
For many applications where high torque is required, however, space is limited. For example, on a torpedo, actuator control space for torpedo steering may be limited to a cylindrical volume of less than 26 cubic inches, but require a torque capacity of 400 inch pounds. Conventional induction actuators do not have a torque density sufficient to meet these specifications.